The present invention relates to a cascode amplifier composed of a grounded-gate FET or a grounded-base transistor and a grounded-emitter transistor.
An example of the cascode amplifier applied to the final stage of a driver circuit for a color picture tube is disclosed in U.S. Pat. No. 4,203,131. The driver circuit for the picture tube is required to exhibit a wide band frequency characteristic.
In an output transistor, an improved high frequency characteristic is usually accompanied with a correspondingly lowered voltage withstanding capability and decreased collector loss. Accordingly, in the cascode amplifier for which a wide band frequency characteristic is required, the operating voltage has to be set at a low level within a range which brings about no trouble in driving the picture tube. To this end, a bias current which flows through a load resistor of the grounded-base or grounded-gate transistor and corresponds to the black level displayed on the picture tube must be set as small as possible. On the other hand, since the current flowing through the load resistor is substantially equal to the collector current of the grounded-emitter transistor, the biasing point is set in a non-linear rise-up region of a base-emitter voltage (V.sub.BE)-collector current (I.sub.C) characteristic curve for the grounded-emitter transistor, when the bias current is set at a small value. As the consequence, when the bias current is to be decreased, the output signal of the cascode amplifier will directly undergo the influence of non-linear distortion of the V.sub.BE -I.sub.C characteristic. Further, because the cut-off frequency of the grounded-emitter transistor becomes higher as the collector current thereof increases, the setting of the bias current at a small value will lower the cut-off frequency, providing an obstacle in realizing a cascode amplifier of a broad bandwidth.